Hach DR/700 Procedures Manual page 513

FLUORIDE,
Sodium Hexametaphosphate
Sulfate
SPADNS Reagent contains enough arsenite to eliminate interference up to
5 mg/L chlorine. For higher chlorine levels, add one drop of Sodium
Arsenite Solution to 25 mL of sample for each 2 mg/L of chlorine.
To check for interferences from aluminum, read the concentration one
minute after reagent addition, then again after 15 minutes. An appreciable
increase in concentration suggests aluminum interference. Waiting two
hours before making the final reading will eliminate the effect of up to
3.0 mg/L aluminum.
DISTILLATION PROCEDURE
Most interferences can be eliminated by distilling the sample from an acid
solution as described below:
a) Set up the distillation apparatus for the general purpose distillation. See
the Hach Distillation Apparatus Manual. Turn on the water and make
certain it is flowing through the condenser.
b) Measure 100 mL of sample into the distillation flask. Add a magnetic
stirring bar and turn on the heater power switch. Turn the stir control to 5.
Cautiously measure 150 mL of StillVer Distillation Solution (2:1 Sulfuric
Acid) into the flask. If high levels of chloride are present, add 5 mg silver
sulfate for each mg/L chloride present.
c) Turn the heat control to setting 10, with the thermometer in place. The
yellow pilot lamp shows when the heater is on.
d) When the temperature reaches 180 °C (about one hour), turn the still
off. Analyze the distillate by the above method.
SUMMARY OF METHOD
The SPADNS Method for fluoride determination involves the reaction of
fluoride with a red zirconium-dye solution. The fluoride combines with
part of the zirconium to form a colorless complex, thus bleaching the red
color in an amount proportional to the fluoride concentration. This
method is approved by the USEPA for NPDES and NPDWR reporting
purposes when the samples have been distilled. Seawater and wastewater
samples require distillation. See Optional Apparatus for distillation
apparatus listing.
continued
1.0 mg/L
200 mg/L
57-7
-
+0.1 mg/L F
-
+0.1 mg/L F